Donors are given injections of a hormone called G-CSF every day for 5 days to stimulate release of stem cells from the bone marrow into the bloodstream for collection. On the day of the last injection, donors undergo apheresis to obtain white cells and stem cells. For this procedure, blood is withdrawn through a catheter (plastic tube) placed in a vein and directed into a machine where the white cells and stem cells are separated from the rest of the blood by a spinning process. These cells are extracted and collected in a bag inside the machine, and the rest of the blood is returned to the donor through a second catheter in a vein in the other arm. The procedure takes 4 to 5 hours.